The thermoelectric material is a material which is capable of converting directly thermal energy into electric energy, and its conversion efficiency has a correlation with the following dimensionless figure of merit ZT.ZT=[(σ×S2)/κ]×T=[PF/κ]×T (σ: Electric conductivity, S: Seebeck coefficient, κ: Thermal conductivity, T: Absolute temperature)
To increase this ZT, the thermal conductivity κ needs to be decreased.
The filled skutterudite (RxCo4Sb12)-based material (0<x≦1) is a kind of thermoelectric material having a crystal structure in which a cage formed by Co and Sb is filled with a filling element R in a central portion thereof. The filling element R charged in the cage vibrates at a natural frequency, and is capable of decreasing the thermal conductivity κ by scattering lattice vibration resonantly (rattling effect). Moreover, Co4Sb12 containing no filling element R is a p-type thermoelectric material, but the filling element R is an electron donor dopant. Accordingly, filled skutterudite RxCo4Sb12 is an n-type semiconductor.
As these filling elements R, various elements such as alkali metal elements (Non-patent Literature 1), alkaline earth metal elements (Non-patent Literature 2), rare earth elements (Patent Literature 1), group IIIB elements (Patent Literature 2), and other elements (Non-patent Literatures 3 to 5) have been suggested. In addition, as the number of filling elements is increased from one to two or three (multiple filling), the thermal conductivity κ decreases, and therefore ZT can be increased. In general, high-performance n-type multiple-filled skutterudite does not contain Fe which substitutes Co site, has an amount x of the filling element of 0.3 or lower, and has the number of the filling element of three or lower (Non-patent Literature 6).
Fe-substituted system, for example, has been reported in Patent Literature 3. However, the filling elements R are only Yb and alkaline earth metal elements, and the amount x of the filling element of the composition which is specifically disclosed as an Example is 0.4 or lower.
Patent Literature 4 discloses an n-type skutterudite material which contains five kinds of filling elements R (Ca, Yb, Al, Ga, In) and has the amount x of the filling element of 0.4 to 0.9, and in which some of Co sites have been substituted with Fe.
That is, there is no example of reports concerning an n-type thermoelectric material including a filled skutterudite-based compound in which more than five kinds of the filling elements are contained, the filling elements R are added at a high concentration and the concentration of a carrier has been adjusted with Fe.